1. Field of the Invention
The present invention relates to a magnetic recording medium such as a magnetic tape and a magnetic disk.
2. Description of the Related Art
A magnetic recording medium generally used such as a magnetic tape and a floppy disk is produced by applying a magnetic coating composition onto a substrate such as polyethylene terephthalate film in order to form a magnetic layer. The magnetic coating composition is usually prepared by dispersing needle-like magnetic particles having a major axis with a length of 1 .mu.m or less in a resinous binder together with appropriate additives such as dispersing agents, lubricating agents, and antistatic agents.
The resinous binder contained in the magnetic layer is required to disperse magnetic particles excellently, to contain a great amount of magnetic particles, and to orientate magnetic particles contained in the binder. Also, the resinous binder is required to provide a magnetic layer with excellent durability, abrasion resistance, heat resistance, and adhesion to a non-magnetic support. Thus, the resinous binder plays a very important role.
Examples of conventional resinous binders contained in the magnetic layer include mixtures of polyurethane resins of an adipate type or a polycaprolactone type and nitrocellulose or polyvinyl chloride type copolymers. The improvement of the dispersibility of the magnetic particles is disclosed in Japanese Patent Publication No. 58-41565, describing that it is effective to use a polyurethane in which a metal sulfonate group is introduced.
In the magnetic recording medium, in order to improve the S/N ratio (signal/noise ratio) and realize a high recording density, the following measures are taken: More finely ground magnetic particles are packed in the magnetic layer and a high orientation is obtained; and a smooth surface of the magnetic layer is achieved.
However, in the case where the magnetic recording medium with a smooth surface is used as a magnetic tape, as the surface of the magnetic layer becomes smoother, the adhesion between the magnetic layer and the magnetic head is increased, resulting in an increase in the coefficient of friction. Namely, friction is increased. As a result, the running property of the magnetic tape and the running durability thereof are deteriorated. For this reason, resinous binders which have satisfactory durability and abrasion resistance have been required. In addition, resinous binders which have satisfactory heat resistance and adhesion with respect to non-magnetic supports have been required. Conventional resinous binders do not satisfy these conditions.
A polyurethane resin containing an aromatic polyester diol as a high molecular weight polyol is excellent in mechanical strength, abrasion resistance, heat resistance, etc. and overcomes some of the abovementioned problems; however, the other problems are not overcome yet. The polyurethane resin containing an aromatic polyester diol as a high molecular weight polyol has a higher glass transition temperature, compared with conventional polyurethane resins. Thus, a magnetic recording medium with a magnetic layer having the polyurethane resin containing an aromatic polyester diol is subjected to a calender treatment, the surface of the layer is not likely to be smooth. Furthermore, the adhesion between the magnetic layer and the non-magnetic support is not sufficient. The poor adhesion sometimes causes peeling of the magnetic layer from the non-magnetic support at the ends thereof, when a magnetic tape with the magnetic layer formed on the non-magnetic support is slit. When the magnetic layer is peeled off, the magnetic particles contained therein drop. The dropped magnetic particles adhere to a magnetic head, so that a dropout (i.e., lack of signals in the magnetic recording medium) arises.
In order to improve the S/N ratio and realize a high recording density in the magnetic recording medium, it is required that the magnetic particles with a high Hc (resistance to magnetic force) such as metal magnetic particles are packed in the magnetic layer, and that the magnetic particles should be oriented in a higher degree. In the case of the conventional resinous binders, it is difficult to disperse these magnetic particles therein. Insufficient dispersion of the particles increases porosity in the magnetic layer, as well as decreases electromagnetic conversion characteristics. The increase in the porosity deteriorates running durability of the obtained magnetic recording medium. The introduction of a metal sulfonate group into the polyurethane is remarkably effective for the improvement of dispersibility of the magnetic particles in the magnetic layer; however, the introduction of the metal sulfonate group sometimes increases the viscosity of a magnetic coating composition, resulting in the decrease in productivity of magnetic recording media to be obtained.